Did you have your system Diagnosed and are you sure that it is an expansion valve problem? Because it is very important to have the system properly inspected before you start replacing parts, which can be very expensive.

here is a link to repair info for your truck, including expansion valve installation
http://bit.ly/mitchell_repair_manuals_online

check you can see refrigerant in the sight glass on the reciaver drier when the system is on . sometimes under load and low rpm the a/c system will shut off to prevent the engine stalling . check if it works around town then on over a constant distance